Poly(isobutylene-co-isoprene) or IIR, is a synthetic elastomer commonly known as butyl rubber (or butyl polymer) which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (usually not more than 2.5 mol %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, usually not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Halogenation of this butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company (c) 1987), pp. 297-300.
The development of halogenated butyl rubber (halobutyl, or XIIR) has greatly extended the usefulness of butyl by providing much higher curing rates and enabling co-vulcanization with general purpose rubbers such as natural rubber and styrene-butadiene rubber. Butyl rubber and halobutyl rubber are high value polymers, as they possess the unique combination of properties (for example, excellent impermeability, good flex, good weatherability, co-vulcanization with high unsaturation rubbers, in the case of halobutyl). These properties allowed the development of more durable tubeless tires with the air retaining inner liner chemically bonded to the body of the tire.
In addition to tire applications, the good impermeability, weathering resistance, ozone resistance, vibration dampening, and stability of halobutyl rubbers make them good candidates for materials for pharmaceutical stoppers, construction sealants, hoses, and mechanical goods.
Like other rubbers, for most applications, halobutyl rubber must be compounded and vulcanized (chemically crosslinked) to yield useful, durable end use products. The selection and ratios of the proper fillers, processing aids, stabilizers, and curatives also play critical roles in both how the compound will process and how the end product will behave.
Elemental sulfur and organic accelerators are widely used to crosslink butyl rubber. The low level of unsaturation requires aggressive accelerators such as thiuram or thiocarbamates. The vulcanization proceeds at the isoprene site with the polysulfidic cross links attached at the allylic positions, displacing the allylic hydrogen. The number of sulfur atoms per crosslink is between one and four or more. Cure rate and cure state both increase if the diolefin content is increased resulting in higher degree of unsaturation. Sulfur cross-links have limited stability at sustained high temperature.
Resin cure systems which commonly use alkyl phenol-formaldehyde derivatives provide for carbon-carbon cross-links and more stable compounds.
In halobutyl rubber, the existence of allylic halogen allows easier cross-linking than allylic hydrogen due to the fact that halogen is a better leaving group in nucleophilic substitution reactions. Furthermore, bromobutyl is faster curing than chlorobutyl and has better adhesion to high unsaturation rubbers.
Existing prior art systems for the cure of bromobutyl and bromine containing polymers generally use sulfur and zinc derivatives as curing agent.
For example, to improve the physical characteristics of tire liner compositions comprised of blends of halobutyl rubber and epihalohydrin rubber, it was disclosed in U.S. Pat. No. 4,591,617 to crosslink the tire liner compositions with a crosslinking composition containing both (1) a sulfur curative system, which cures through the unsaturation present in the halobutyl rubber or mixtures thereof with butyl rubber, and (2) a nonsulfur curative system, which cures through the halogen functionality of the epihalohydrin rubber in the blend. The sulfur curative system disclosed comprises (a) sulfur, (b) a conventional sulfur accelerator, such as mercaptobenzothiazole and its derivatives, sulfenamides, thiurams, and dithiocarbamate salts, and (c) a zinc oxide promotor. The nonsulfur curative system disclosed comprises di- and tri-functional mercapto compounds and their derivatives, such as 2,5-dimercapto-1,3,4-thiadiazole or trithiocyanuric acid, alone or in combination with a basic activator as set forth in U.S. Pat. Nos. 4,128,510 and 4,288,576.
The basic activator materials that are disclosed in U.S. Pat. Nos. 4,128,510 and 4,288,576 include basic amines and amine salts, and basic metal oxides and hydroxides and their salts with weak acids, such as, for example, lead oxides, zinc oxide, magnesium oxide, calcium oxide, calcium hydroxide, barium oxide, zinc carbonate, barium carbonate, sodium carbonate, lead acetate and sodium acetate. These basic materials are disclosed as being suitable for use in combination with certain 2,5-dimercapto-1,3,4-thiadiazoles as a crosslinking system for halogen-containing polymers, including epihalohydrin homopolymers and copolymers, chlorobutyl rubber and bromobutyl rubber.
Another cure system for crosslinking halogen-containing rubbers is disclosed in U.S. Pat. No. 4,357,446 and comprises (1) 2,3-dimercapto-pyrazine or quinoxaline compound as a crosslinking agent, and (2) a compound of a metal of Group II or IV as an acid acceptor. The acid acceptors disclosed include oxides, hydroxides, carbonates, carboxylates silicates, borates and phosphites of Group II or IV metals; and oxides, basic carbonates, basic carboxylates, basic phosphites, basic sulfites, and tribasic sulfates of Group IVa metals.
The existing prior art cure systems for bromobutyl and bromine containing polymers typically contain sulfur and zinc oxides, which are “dirty”, i.e., with high extractable levels of sulfur and zinc oxides, and are unsuitable, or unacceptable for various pharmaceutical applications.
Therefore, there remains a need for a clean cure system free of sulphur and zinc oxide for bromobutyl and bromine containing polymers.
The present invention addresses the afore-mentioned problem by providing a new class of sulfur free and zinc oxide free cure system which is based on bisphosphine derivatives for curing bromobutyl and bromine containing polymers. These new and novel crosslinkers contain multifunctional phosphine groups which react readily with the allylic bromide group on the polymers via nucleophilic substitution to form an extensive covalent crosslinking network with ionomer formation.
The approach disclosed in the present invention attempts to solve the existing problems associated with sulphur, zinc oxide and other agents for the curing of bromobutyl and bromine containing polymers. This is of much interest to the industry since the cure system of the invention herein is clean and has a minimum of chemicals added to the rubber matrix to obtain a cure.
There have been some recent efforts in exploring cure systems for halobutyl which are free of sulfur and zinc oxide. For example, in a journal article by Parent et al. in Polymer 2011, 52(24), 5410-5418, it describes a new class of elastomeric ionomers involving the use of dialkylated imidazoles as cross-linkers for bromobutyl rubber.
The journal article also provided only one example of the use of a bisphosphine agent, namely 1,2-bis(diphenylphosphino)ethane (DIPHOS) to cross-link bromobutyl rubber. The authors compared the cure behaviour of DIPHOS to the bis-imidazole and commented that the DIPHOS agent was too reactive at 100° C. However, the article did not provide any results of a sufficient induction period at 160° C. No other bisphosphine agents besides DIPHOS were mentioned in this article. The article also fails to recognize the novel aspect of the bisphosphine agent in which the length of the alkyl spacer between the phosphine moieties plays a crucial role in the curing chemistry of bromobutyl rubber.
The present invention discloses however, that DIPHOS, as shown in the prior art, is not representative of the chemistry for this class of crosslinking agent. Instead, the better choice of the bisphosphine agent to achieve optimum cross-linking density is where the alkyl spacer is consisted of three methylene chain or longer.
The cure behaviour and cure properties can be further optimized through the judicious choice of the bisphosphine agent. Replacing the alkyl spacer with an aromatic spacer between the phosphine groups can alter the cure rate and the state of cure.
The present invention offers a method to cure halobutyl rubber with only adding one component (bisphosphine) during the mixing process, followed by heating to obtain the crosslinking.
The chemistry disclosed in the present invention additionally offers the potential for low leachable cured butyl polymers. It provides an advantage in that it does not require the use of peroxides. As a bisphosphine, even if one end gets oxidized, the other end statistically has a good chance of attaching to the elastomer through formation of the ionomer. This will greatly reduce any leachables of the bisphosphine component from the cross-linked polymer network.
Therefore, the present invention offers a more suitable choice of bisphosphine agents as a new class of sulfur free and zinc oxide free cure system for curing halobutyl polymers.